Screens used in the papermaking process, for example dryer screens, are subject to the harsh chemical environment of the papermaking process. Accordingly, such screens are degraded in relatively short periods of time as a result of hydrolytic attack. This causes screen failure and requires frequent replacement of the screen, which results in down time, i.e., increased operating costs, to the paper manufacturer.
Dryer screens currently in use are made predominantly of polyethylene terephthalate (PET). PET is a good material, but improvements can be made. To this end, some manufacturers of materials for papermaking dryer screens have investigated the use of polyphenylene sulfide (PPS) monofilaments. For example, see U.S. Pat. Nos. 4,610,916; 4,748,077; and 4,801,492. While these patents disclose PPS monofilaments which, when compared to PET monofilaments for the same end use, show some performance advantages, the cost of the dryer screens produced from that material is significantly higher.
Accordingly, the search for improved materials that can be used in the manufacture of papermaking dryer screens, among other things, continues. Disclosed hereinafter is a material, poly(2-methyl-1,5-pentylene) terephthalamide, which has good resistance to hydrolytic attack, can be formed into monofilaments, and which is less expensive than PPS monofilaments.
Poly (2-methyl-1,5-pentylene) terephthalamide is known. See U.S. Pat. No. 4,163,101 and Japanese Kokoku No. 19551 (1969). Poly (2-methyl-1,5-pentylene) terephthalamide is also referred to as: 2-methyl pentamethylene terephthalamide; methyl pentamethylene terephthalamide; and M5T.
It is generally known that polyamides may be made from aqueous nylon salt solutions by heating the solution to a temperature from 210.degree. C. to 220.degree. C. and to a pressure of about 18 bars, thereby producing a low molecular weight precondensate. Thereafter, the pressure on the precondensate is lowered and the temperature simultaneously increased to about 270.degree. C. until the desired molecular weight is achieved. See U.S. Pat. No. 4,465,821.
Japanese Kokoku No. 19551 (1969) is directed to a high-elasticity polyamide produced from terephthalic acid and methyl pentamethylene diamine. The polyamide is produced by combining 50 grams of the nylon salt derived from 2-methyl pentamethylene diamine and terephthalic acid with 2.5 cc of water in a test tube. The atmosphere of the test tube is substituted with oxygen and the tube is sealed. Then, the contents of the tube are heated at 230.degree. C. for 4 hours. The resulting reaction product is then immersed in 50 cc of distilled water for over 2 hours and, thereafter, suction-filtered and dried. Finally, the dried, filtered reaction product is polymerized at 285.degree.-229.degree. C. at normal pressures for over one hour, and then at a reduced pressure (3 mm Hg) for over 2 hours. This material has a relative viscosity of 2.58 (in 98% sulfuric acid). The reaction product, when directly polymerized has a relative viscosity of 1.61. This process, however, has been characterized as impractical in an economical sense because of its complexity and its relatively low production yield. See U.S. Pat. No. 4,163,101.
U.S. Pat. No. 4,163,101 is directed to a process for making polyamides, particularly such polyamides as poly (2-methyl pentamethylene terephthalamide) In this process, an aqueous solution of the nylon salt and water soluble, low molecular weight oligoamides is heated from 130.degree. C.-150.degree. C. to the polycondensation temperature of 250.degree. C. to 300.degree. C. at normal (atmospheric) pressure. The aqueous solution of nylon salt and oligoamides is produced by reacting equivalent amounts of dimethyl terephthalate (DMT) with an alkylpentamethylene diamine, such as 2-methylpentamethylene diamine, in the presence of 45 to 100 parts of water per 100 parts of DMT at 90.degree.-100.degree. C. over a period of 5 to 10 hours while distilling out the methanol by-product.
U.S. Pat. No. 4,465,821 is directed to a continuous, normal (atmospheric) pressure process for the production of polyamides, but the production of poly(2-methyl-1,5-pentylene) terephthalamide is not disclosed. In this process, an aqueous solution of the nylon salts derived from equal molar amounts of diamines and dicarboxylic acids is continuously introduced into the precondensate melt of the resultant polyamide. This precondensate melt is maintained at atmospheric pressure and at a temperature of at least 180.degree. C. while water is continuously distilled away.